The invention relates to a method of reducing the circulation of salts, particularly in cement kilns, wherein a partial gas stream charged with harmful substances is removed from the kiln through a bypass conduit; these kiln gases are then cooled and returned to the manufacturing process.
Substances employed in the production of cement contain ancillary components such as alkalis, sulphur, and chlorine, which partially evaporate in the region of the sintering zone and establish circulations that considerably interfere with kiln operation due to the formation of deposits.
These problems occur in particular if, for example, waste materials are employed as substitute raw materials and fuels. These substances may contain large amounts of the above-mentioned ancillary components so that it is necessary to reduce the circulation of salts in such cases. In the prior art, bypass systems are generally employed for this purpose which operate according to the following principle:
removal of a partial gas stream at the point of transition from the kiln to the calcinator or raw meal heat exchanger; PA0 cooling of these kiln gases with fresh air to about 400.degree. C.; PA0 further cooling of the gases with water to about 150.degree. C.; PA0 precipitation of the dust particles in a filter and removal of the dust from the calcination process. PA0 low dust losses due to dust separation in at least two stages; PA0 low heat requirement of the kiln system since the return of the gas into the heat exchanger results in the utilization of the heat to pre-heat the raw meal.
The drawbacks of the application of this technique are the increase in the heat requirement of the system and losses of raw material created by the development of dust. A particular drawback is the installation of such bypass devices in furnace systems which circulate large amounts of dust, as is the case, for example, for heat exchangers operating according to the shaft pre-heating principle.
European Application EP-A-0,154,281 discloses a method for producing cement and an apparatus operating according to the method in which raw material is fired into clinker, the clinker is cooled and ground into a final meal-like cement product. Substances ineterfering with the calcination process, such as alkalis, chlorides and/or heavy metals, are branched off from the calcination process by means of a partial gas extraction (bypass) at least until they reach a point of no longer being harmful. With the addition of additives, an intermediate product is produced from the clinker, with the gas obtained in the partial gas extraction and/or substances contained therein being introduced at least in part into the intermediate product. The intermediate product is ground into the final cement product. As already mentioned above, the branching off of a partial stream of gas ultimately does not prevent the development of salt circulation, rather the resulting intermediate products and their contaminants are integrated in the end product. Depending on the type of cement, this measure may result in problems during further processing.
European Application EP-A-0,138,057 discloses a method, and a system operating according to the method, for producing cement clinker that is low in harmful substances, particularly low in alkalis. Here, the reaction gas stream and the exhaust gas stream of the clinker stage are combined into a single suspension gas stream before being introduced into the lowermost heat exchanger zone of the pre-heating stage. This method is also unable to prevent the effect of salt circulation formation.
U.S. Pat. No. 3,288,450 discloses a method as well as a system for the production of cement. The kiln exhaust gas is initially introduced into a cyclone. By way of a further conduit and a further cyclone, the exhaust gas stream reaches a stack pre-heater which cooperates with further cyclone stages. The material is charged in in the upper region of the stack and passes through the respective cyclone stages where it is to be pre-heated before it is introduced into the region of the rotary kiln. The dust laden exhaust gas stream is conducted through an electrofilter before it is discharged to the ambient air. From the electrofilter, a conduit leads to the part of the kiln exhaust gas conduit disposed between the first two cyclone stages upstream of the stack. By way of a further conduit, cold powdered material may be added from a container. By adding the materials from the electrofilter and/or the container into the waste gas stream, the latter is cooled suddenly, with salt compounds condensing or subliming, respectively, on the dust or material particles. This dust is separated in the first cyclone stage and fed into the rotary kiln. The drawback noted in connection with this process is that the entire exhaust gas stream is cooled suddenly which has an adverse effect on the subsequent precalcining process and on the pre-heating of the raw materials since a sufficient temperature level no longer exists. The result of this is, on the one hand, unsatisfactory precalcination and an increased heat requirement of the kiln system connected therewith.